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<h1>ZooKeeper Programmer's Guide</h1>
<h3>Developing Distributed Applications that use ZooKeeper</h3>
<div id="front-matter">
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#_introduction">Introduction</a>
</li>
<li>
<a href="#ch_zkDataModel">The ZooKeeper Data Model</a>
<ul class="minitoc">
<li>
<a href="#sc_zkDataModel_znodes">ZNodes</a>
<ul class="minitoc">
<li>
<a href="#sc_zkDataMode_watches">Watches</a>
</li>
<li>
<a href="#Data+Access">Data Access</a>
</li>
<li>
<a href="#Ephemeral+Nodes">Ephemeral Nodes</a>
</li>
<li>
<a href="#Sequence+Nodes+--+Unique+Naming">Sequence Nodes -- Unique Naming</a>
</li>
<li>
<a href="#Container+Nodes">Container Nodes</a>
</li>
<li>
<a href="#TTL+Nodes">TTL Nodes</a>
</li>
</ul>
</li>
<li>
<a href="#sc_timeInZk">Time in ZooKeeper</a>
</li>
<li>
<a href="#sc_zkStatStructure">ZooKeeper Stat Structure</a>
</li>
</ul>
</li>
<li>
<a href="#ch_zkSessions">ZooKeeper Sessions</a>
</li>
<li>
<a href="#ch_zkWatches">ZooKeeper Watches</a>
<ul class="minitoc">
<li>
<a href="#sc_WatchSemantics">Semantics of Watches</a>
</li>
<li>
<a href="#sc_WatchRemoval">Remove Watches</a>
</li>
<li>
<a href="#sc_WatchGuarantees">What ZooKeeper Guarantees about Watches</a>
</li>
<li>
<a href="#sc_WatchRememberThese">Things to Remember about Watches</a>
</li>
</ul>
</li>
<li>
<a href="#sc_ZooKeeperAccessControl">ZooKeeper access control using ACLs</a>
<ul class="minitoc">
<li>
<a href="#sc_ACLPermissions">ACL Permissions</a>
<ul class="minitoc">
<li>
<a href="#sc_BuiltinACLSchemes">Builtin ACL Schemes</a>
</li>
<li>
<a href="#ZooKeeper+C+client+API">ZooKeeper C client API</a>
</li>
</ul>
</li>
</ul>
</li>
<li>
<a href="#sc_ZooKeeperPluggableAuthentication">Pluggable ZooKeeper authentication</a>
</li>
<li>
<a href="#ch_zkGuarantees">Consistency Guarantees</a>
</li>
<li>
<a href="#ch_bindings">Bindings</a>
<ul class="minitoc">
<li>
<a href="#Java+Binding">Java Binding</a>
<ul class="minitoc">
<li>
<a href="#sc_java_client_configuration">Client Configuration Parameters</a>
</li>
</ul>
</li>
<li>
<a href="#C+Binding">C Binding</a>
<ul class="minitoc">
<li>
<a href="#Installation">Installation</a>
</li>
<li>
<a href="#Building+Your+Own+C+Client">Building Your Own C Client</a>
</li>
</ul>
</li>
</ul>
</li>
<li>
<a href="#ch_guideToZkOperations">Building Blocks: A Guide to ZooKeeper Operations</a>
<ul class="minitoc">
<li>
<a href="#sc_errorsZk">Handling Errors</a>
</li>
<li>
<a href="#sc_connectingToZk">Connecting to ZooKeeper</a>
</li>
<li>
<a href="#sc_readOps">Read Operations</a>
</li>
<li>
<a href="#sc_writeOps">Write Operations</a>
</li>
<li>
<a href="#sc_handlingWatches">Handling Watches</a>
</li>
<li>
<a href="#sc_miscOps">Miscelleaneous ZooKeeper Operations</a>
</li>
</ul>
</li>
<li>
<a href="#ch_programStructureWithExample">Program Structure, with Simple Example</a>
</li>
<li>
<a href="#ch_gotchas">Gotchas: Common Problems and Troubleshooting</a>
</li>
</ul>
</div>
</div>
  

  

  

  
<a name="_introduction"></a>
<h2 class="h3">Introduction</h2>
<div class="section">
<p>This document is a guide for developers wishing to create
    distributed applications that take advantage of ZooKeeper's coordination
    services. It contains conceptual and practical information.</p>
<p>The first four sections of this guide present higher level
    discussions of various ZooKeeper concepts. These are necessary both for an
    understanding of how ZooKeeper works as well how to work with it. It does
    not contain source code, but it does assume a familiarity with the
    problems associated with distributed computing. The sections in this first
    group are:</p>
<ul>
      
<li>
        
<p>
<a href="#ch_zkDataModel">The ZooKeeper Data Model</a>
</p>
      
</li>

      
<li>
        
<p>
<a href="#ch_zkSessions">ZooKeeper Sessions</a>
</p>
      
</li>

      
<li>
        
<p>
<a href="#ch_zkWatches">ZooKeeper Watches</a>
</p>
      
</li>

      
<li>
        
<p>
<a href="#ch_zkGuarantees">Consistency Guarantees</a>
</p>
      
</li>
    
</ul>
<p>The next four sections provide practical programming
    information. These are:</p>
<ul>
      
<li>
        
<p>
<a href="#ch_guideToZkOperations">Building Blocks: A Guide to ZooKeeper Operations</a>
</p>
      
</li>

      
<li>
        
<p>
<a href="#ch_bindings">Bindings</a>
</p>
      
</li>

      
<li>
        
<p>
<a href="#ch_programStructureWithExample">Program Structure, with Simple Example</a>
        <em>[tbd]</em>
</p>
      
</li>

      
<li>
        
<p>
<a href="#ch_gotchas">Gotchas: Common Problems and Troubleshooting</a>
</p>
      
</li>
    
</ul>
<p>The book concludes with an <a href="#apx_linksToOtherInfo">appendix</a> containing links to other
    useful, ZooKeeper-related information.</p>
<p>Most of information in this document is written to be accessible as
    stand-alone reference material. However, before starting your first
    ZooKeeper application, you should probably at least read the chaptes on
    the <a href="#ch_zkDataModel">ZooKeeper Data Model</a> and <a href="#ch_guideToZkOperations">ZooKeeper Basic Operations</a>. Also,
    the <a href="#ch_programStructureWithExample">Simple Programmming
    Example</a> <em>[tbd]</em> is helpful for understanding the basic
    structure of a ZooKeeper client application.</p>
</div>

  
<a name="ch_zkDataModel"></a>
<h2 class="h3">The ZooKeeper Data Model</h2>
<div class="section">
<p>ZooKeeper has a hierarchal name space, much like a distributed file
    system. The only difference is that each node in the namespace can have
    data associated with it as well as children. It is like having a file
    system that allows a file to also be a directory. Paths to nodes are
    always expressed as canonical, absolute, slash-separated paths; there are
    no relative reference. Any unicode character can be used in a path subject
    to the following constraints:</p>
<ul>
      
<li>
        
<p>The null character (\u0000) cannot be part of a path name. (This
        causes problems with the C binding.)</p>
      
</li>

      
<li>
        
<p>The following characters can't be used because they don't
        display well, or render in confusing ways: \u0001 - \u001F and \u007F
        - \u009F.</p>
      
</li>

      
<li>
        
<p>The following characters are not allowed: \ud800 - uF8FF,
        \uFFF0 - uFFFF.</p>
      
</li>

      
<li>
        
<p>The "." character can be used as part of another name, but "."
        and ".." cannot alone be used to indicate a node along a path,
        because ZooKeeper doesn't use relative paths. The following would be
        invalid: "/a/b/./c" or "/a/b/../c".</p>
      
</li>

      
<li>
        
<p>The token "zookeeper" is reserved.</p>
      
</li>
    
</ul>
<a name="sc_zkDataModel_znodes"></a>
<h3 class="h4">ZNodes</h3>
<p>Every node in a ZooKeeper tree is referred to as a
      <em>znode</em>. Znodes maintain a stat structure that
      includes version numbers for data changes, acl changes. The stat
      structure also has timestamps. The version number, together with the
      timestamp, allows ZooKeeper to validate the cache and to coordinate
      updates. Each time a znode's data changes, the version number increases.
      For instance, whenever a client retrieves data, it also receives the
      version of the data. And when a client performs an update or a delete,
      it must supply the version of the data of the znode it is changing. If
      the version it supplies doesn't match the actual version of the data,
      the update will fail. (This behavior can be overridden. For more
      information see... )<em>[tbd...]</em>
</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
        
<p>In distributed application engineering, the word
        <em>node</em> can refer to a generic host machine, a
        server, a member of an ensemble, a client process, etc. In the ZooKeeper
        documentation, <em>znodes</em> refer to the data nodes.
        <em>Servers</em>  refer to machines that make up the
        ZooKeeper service; <em>quorum peers</em> refer to the
        servers that make up an ensemble; client refers to any host or process
        which uses a ZooKeeper service.</p>
      
</div>
</div>
<p>Znodes are the main enitity that a programmer access. They have
      several characteristics that are worth mentioning here.</p>
<a name="sc_zkDataMode_watches"></a>
<h4>Watches</h4>
<p>Clients can set watches on znodes. Changes to that znode trigger
        the watch and then clear the watch. When a watch triggers, ZooKeeper
        sends the client a notification. More information about watches can be
        found in the section 
	    <a href="#ch_zkWatches">ZooKeeper Watches</a>.</p>
<a name="Data+Access"></a>
<h4>Data Access</h4>
<p>The data stored at each znode in a namespace is read and written
        atomically. Reads get all the data bytes associated with a znode and a
        write replaces all the data. Each node has an Access Control List
        (ACL) that restricts who can do what.</p>
<p>ZooKeeper was not designed to be a general database or large
        object store. Instead, it manages coordination data. This data can
        come in the form of configuration, status information, rendezvous, etc.
        A common property of the various forms of coordination data is that
        they are relatively small: measured in kilobytes.
        The ZooKeeper client and the server implementations have sanity checks
        to ensure that znodes have less than 1M of data, but the data should
        be much less than that on average. Operating on relatively large data
        sizes will cause some operations to take much more time than others and
        will affect the latencies of some operations because of the extra time
        needed to move more data over the network and onto storage media. If
        large data storage is needed, the usually pattern of dealing with such
        data is to store it on a bulk storage system, such as NFS or HDFS, and
        store pointers to the storage locations in ZooKeeper.</p>
<a name="Ephemeral+Nodes"></a>
<h4>Ephemeral Nodes</h4>
<p>ZooKeeper also has the notion of ephemeral nodes. These znodes
        exists as long as the session that created the znode is active. When
        the session ends the znode is deleted. Because of this behavior
        ephemeral znodes are not allowed to have children.</p>
<a name="Sequence+Nodes+--+Unique+Naming"></a>
<h4>Sequence Nodes -- Unique Naming</h4>
<p>When creating a znode you can also request that
        ZooKeeper append a monotonically increasing counter to the end
        of path. This counter is unique to the parent znode. The
        counter has a format of %010d -- that is 10 digits with 0
        (zero) padding (the counter is formatted in this way to
        simplify sorting), i.e. "&lt;path&gt;0000000001". See
        <a href="recipes.html#sc_recipes_Queues">Queue
        Recipe</a> for an example use of this feature. Note: the
        counter used to store the next sequence number is a signed int
        (4bytes) maintained by the parent node, the counter will
        overflow when incremented beyond 2147483647 (resulting in a
        name "&lt;path&gt;-2147483648").</p>
<a name="Container+Nodes"></a>
<h4>Container Nodes</h4>
<p>
<strong>Added in 3.5.3</strong>
</p>
<p>ZooKeeper has the notion of container znodes. Container znodes are
          special purpose znodes useful for recipes such as leader, lock, etc.
          When the last child of a container is deleted, the container becomes
          a candidate to be deleted by the server at some point in the future.</p>
<p>Given this property, you should be prepared to get
          KeeperException.NoNodeException when creating children inside of
          container znodes. i.e. when creating child znodes inside of container znodes
          always check for KeeperException.NoNodeException and recreate the container
          znode when it occurs.</p>
<a name="TTL+Nodes"></a>
<h4>TTL Nodes</h4>
<p>
<strong>Added in 3.5.3</strong>
</p>
<p>When creating PERSISTENT or PERSISTENT_SEQUENTIAL znodes,
          you can optionally set a TTL in milliseconds for the znode. If the znode
          is not modified within the TTL and has no children it will become a candidate
          to be deleted by the server at some point in the future.</p>
<p>Note: TTL Nodes must be enabled via System property as
        they are disabled by default. See the <a href="zookeeperAdmin.html#sc_configuration">Administrator's
        Guide</a> for details. If you attempt to create TTL Nodes without the proper System property set the server
        will throw <em>KeeperException.UnimplementedException</em>.</p>
<a name="sc_timeInZk"></a>
<h3 class="h4">Time in ZooKeeper</h3>
<p>ZooKeeper tracks time multiple ways:</p>
<ul>
        
<li>
          
<p>
<strong>Zxid</strong>
</p>

          
<p>Every change to the ZooKeeper state receives a stamp in the
          form of a <em>zxid</em> (ZooKeeper Transaction Id).
          This exposes the total ordering of all changes to ZooKeeper. Each
          change will have a unique zxid and if zxid1 is smaller than zxid2
          then zxid1 happened before zxid2.</p>
        
</li>

        
<li>
          
<p>
<strong>Version numbers</strong>
</p>

          
<p>Every change to a node will cause an increase to one of the
          version numbers of that node. The three version numbers are version
          (number of changes to the data of a znode), cversion (number of
          changes to the children of a znode), and aversion (number of changes
          to the ACL of a znode).</p>
        
</li>

        
<li>
          
<p>
<strong>Ticks</strong>
</p>

          
<p>When using multi-server ZooKeeper, servers use ticks to define
          timing of events such as status uploads, session timeouts,
          connection timeouts between peers, etc. The tick time is only
          indirectly exposed through the minimum session timeout (2 times the
          tick time); if a client requests a session timeout less than the
          minimum session timeout, the server will tell the client that the
          session timeout is actually the minimum session timeout.</p>
        
</li>

        
<li>
          
<p>
<strong>Real time</strong>
</p>

          
<p>ZooKeeper doesn't use real time, or clock time, at all except
          to put timestamps into the stat structure on znode creation and
          znode modification.</p>
        
</li>
      
</ul>
<a name="sc_zkStatStructure"></a>
<h3 class="h4">ZooKeeper Stat Structure</h3>
<p>The Stat structure for each znode in ZooKeeper is made up of the
      following fields:</p>
<ul>
        
<li>
          
<p>
<strong>czxid</strong>
</p>

          
<p>The zxid of the change that caused this znode to be
          created.</p>
        
</li>

        
<li>
          
<p>
<strong>mzxid</strong>
</p>

          
<p>The zxid of the change that last modified this znode.</p>
        
</li>

        
<li>
          
<p>
<strong>pzxid</strong>
</p>

          
<p>The zxid of the change that last modified children of this znode.</p>
        
</li>

        
<li>
          
<p>
<strong>ctime</strong>
</p>

          
<p>The time in milliseconds from epoch when this znode was
          created.</p>
        
</li>

        
<li>
          
<p>
<strong>mtime</strong>
</p>

          
<p>The time in milliseconds from epoch when this znode was last
          modified.</p>
        
</li>

        
<li>
          
<p>
<strong>version</strong>
</p>

          
<p>The number of changes to the data of this znode.</p>
        
</li>

        
<li>
          
<p>
<strong>cversion</strong>
</p>

          
<p>The number of changes to the children of this znode.</p>
        
</li>

        
<li>
          
<p>
<strong>aversion</strong>
</p>

          
<p>The number of changes to the ACL of this znode.</p>
        
</li>

        
<li>
          
<p>
<strong>ephemeralOwner</strong>
</p>

          
<p>The session id of the owner of this znode if the znode is an
          ephemeral node. If it is not an ephemeral node, it will be
          zero.</p>
        
</li>

        
<li>
          
<p>
<strong>dataLength</strong>
</p>

          
<p>The length of the data field of this znode.</p>
        
</li>

        
<li>
          
<p>
<strong>numChildren</strong>
</p>

          
<p>The number of children of this znode.</p>
        
</li>

      
</ul>
</div>

  
<a name="ch_zkSessions"></a>
<h2 class="h3">ZooKeeper Sessions</h2>
<div class="section">
<p>A ZooKeeper client establishes a session with the ZooKeeper
    service by creating a handle to the service using a language
    binding. Once created, the handle starts of in the CONNECTING state
    and the client library tries to connect to one of the servers that
    make up the ZooKeeper service at which point it switches to the
    CONNECTED state. During normal operation will be in one of these
    two states. If an unrecoverable error occurs, such as session
    expiration or authentication failure, or if the application explicitly
    closes the handle, the handle will move to the CLOSED state.
    The following figure shows the possible state transitions of a
    ZooKeeper client:</p>
<img alt="" src="images/state_dia.jpg"><p>To create a client session the application code must provide
    a connection string containing a comma separated list of host:port pairs,
    each corresponding to a ZooKeeper server (e.g. "127.0.0.1:4545" or
    "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002"). The ZooKeeper
    client library will pick an arbitrary server and try to connect to
    it. If this connection fails, or if the client becomes
    disconnected from the server for any reason, the client will
    automatically try the next server in the list, until a connection
    is (re-)established.</p>
<p> 
<strong>Added in 3.2.0</strong>: An
    optional "chroot" suffix may also be appended to the connection
    string. This will run the client commands while interpreting all
    paths relative to this root (similar to the unix chroot
    command). If used the example would look like:
    "127.0.0.1:4545/app/a" or
    "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a" where the
    client would be rooted at "/app/a" and all paths would be relative
    to this root - ie getting/setting/etc...  "/foo/bar" would result
    in operations being run on "/app/a/foo/bar" (from the server
    perspective). This feature is particularly useful in multi-tenant
    environments where each user of a particular ZooKeeper service
    could be rooted differently. This makes re-use much simpler as
    each user can code his/her application as if it were rooted at
    "/", while actual location (say /app/a) could be determined at
    deployment time.</p>
<p>When a client gets a handle to the ZooKeeper service,
    ZooKeeper creates a ZooKeeper session, represented as a 64-bit
    number, that it assigns to the client. If the client connects to a
    different ZooKeeper server, it will send the session id as a part
    of the connection handshake.  As a security measure, the server
    creates a password for the session id that any ZooKeeper server
    can validate.The password is sent to the client with the session
    id when the client establishes the session. The client sends this
    password with the session id whenever it reestablishes the session
    with a new server.</p>
<p>One of the parameters to the ZooKeeper client library call
    to create a ZooKeeper session is the session timeout in
    milliseconds. The client sends a requested timeout, the server
    responds with the timeout that it can give the client. The current
    implementation requires that the timeout be a minimum of 2 times
    the tickTime (as set in the server configuration) and a maximum of
    20 times the tickTime. The ZooKeeper client API allows access to
    the negotiated timeout.</p>
<p>When a client (session) becomes partitioned from the ZK
    serving cluster it will begin searching the list of servers that
    were specified during session creation. Eventually, when
    connectivity between the client and at least one of the servers is
    re-established, the session will either again transition to the
    "connected" state (if reconnected within the session timeout
    value) or it will transition to the "expired" state (if
    reconnected after the session timeout). It is not advisable to
    create a new session object (a new ZooKeeper.class or zookeeper
    handle in the c binding) for disconnection. The ZK client library
    will handle reconnect for you. In particular we have heuristics
    built into the client library to handle things like "herd effect",
    etc... Only create a new session when you are notified of session
    expiration (mandatory).</p>
<p>Session expiration is managed by the ZooKeeper cluster
    itself, not by the client. When the ZK client establishes a
    session with the cluster it provides a "timeout" value detailed
    above. This value is used by the cluster to determine when the
    client's session expires. Expirations happens when the cluster
    does not hear from the client within the specified session timeout
    period (i.e. no heartbeat). At session expiration the cluster will
    delete any/all ephemeral nodes owned by that session and
    immediately notify any/all connected clients of the change (anyone
    watching those znodes). At this point the client of the expired
    session is still disconnected from the cluster, it will not be
    notified of the session expiration until/unless it is able to
    re-establish a connection to the cluster. The client will stay in
    disconnected state until the TCP connection is re-established with
    the cluster, at which point the watcher of the expired session
    will receive the "session expired" notification.</p>
<p>Example state transitions for an expired session as seen by
    the expired session's watcher:</p>
<ol>
      
<li>
<p>'connected' : session is established and client
      is communicating with cluster (client/server communication is
      operating properly)</p>
</li>
      
<li>
<p>.... client is partitioned from the
      cluster</p>
</li>
      
<li>
<p>'disconnected' : client has lost connectivity
      with the cluster</p>
</li>
      
<li>
<p>.... time elapses, after 'timeout' period the
      cluster expires the session, nothing is seen by client as it is
      disconnected from cluster</p>
</li>
      
<li>
<p>.... time elapses, the client regains network
      level connectivity with the cluster</p>
</li>
      
<li>
<p>'expired' : eventually the client reconnects to
      the cluster, it is then notified of the
      expiration</p>
</li>
    
</ol>
<p>Another parameter to the ZooKeeper session establishment
    call is the default watcher. Watchers are notified when any state
    change occurs in the client. For example if the client loses
    connectivity to the server the client will be notified, or if the
    client's session expires, etc... This watcher should consider the
    initial state to be disconnected (i.e. before any state changes
    events are sent to the watcher by the client lib). In the case of
    a new connection, the first event sent to the watcher is typically
    the session connection event.</p>
<p>The session is kept alive by requests sent by the client. If
    the session is idle for a period of time that would timeout the
    session, the client will send a PING request to keep the session
    alive. This PING request not only allows the ZooKeeper server to
    know that the client is still active, but it also allows the
    client to verify that its connection to the ZooKeeper server is
    still active. The timing of the PING is conservative enough to
    ensure reasonable time to detect a dead connection and reconnect
    to a new server.</p>
<p>
      Once a connection to the server is successfully established
      (connected) there are basically two cases where the client lib generates
      connectionloss (the result code in c binding, exception in Java -- see 
      the API documentation for binding specific details) when either a synchronous or
      asynchronous operation is performed and one of the following holds:
    </p>
<ol>
      
<li>
<p>The application calls an operation on a session that is no
      longer alive/valid</p>
</li>
      
<li>
<p>The ZooKeeper client disconnects from a server when there
      are pending operations to that server, i.e., there is a pending asynchronous call.
      </p>
</li>
    
</ol>
<p> 
<strong>Added in 3.2.0 -- SessionMovedException</strong>. There is an internal
      exception that is generally not seen by clients called the SessionMovedException.
      This exception occurs because a request was received on a connection for a session
      which has been reestablished on a different server. The normal cause of this error is
      a client that sends a request to a server, but the network packet gets delayed, so
      the client times out and connects to a new server. When the delayed packet arrives at
      the first server, the old server detects that the session has moved, and closes the
      client connection. Clients normally do not see this error since they do not read
      from those old connections. (Old connections are usually closed.) One situation in which this
      condition can be seen is when two clients try to reestablish the same connection using
      a saved session id and password. One of the clients will reestablish the connection
      and the second client will be disconnected (causing the pair to attempt to re-establish
      its connection/session indefinitely).</p>
<p> 
<strong>Updating the list of servers</strong>.  We allow a client to 
      update the connection string by providing a new comma separated list of host:port pairs, 
      each corresponding to a ZooKeeper server. The function invokes a probabilistic load-balancing 
      algorithm which may cause the client to disconnect from its current host with the goal
      to achieve expected uniform number of connections per server in the new list. 
      In case the current host to which the client is connected is not in the new list
      this call will always cause the connection to be dropped. Otherwise, the decision
	  is based on whether the number of servers has increased or decreased and by how much.	
	</p>
<p>
      For example, if the previous connection string contained 3 hosts and now the list contains
      these 3 hosts and 2 more hosts, 40% of clients connected to each of the 3 hosts will
      move to one of the new hosts in order to balance the load. The algorithm will cause the client 
      to drop its connection to the current host to which it is connected with probability 0.4 and in this 
	  case cause the client to connect to one of the 2 new hosts, chosen at random.
    </p>
<p>
	  Another example -- suppose we have 5 hosts and now update the list to remove 2 of the hosts, 
	  the clients connected to the 3 remaining hosts will stay connected, whereas all clients connected 
	  to the 2 removed hosts will need to move to one of the 3 hosts, chosen at random. If the connection
	  is dropped, the client moves to a special mode where he chooses a new server to connect to using the
	  probabilistic algorithm, and not just round robin. 
    </p>
<p>
	  In the first example, each client decides to disconnect with probability 0.4 but once the decision is
	  made, it will try to connect to a random new server and only if it cannot connect to any of the new 
	  servers will it try to connect to the old ones. After finding a server, or trying all servers in the 
	  new list and failing to connect, the client moves back to the normal mode of operation where it picks
	  an arbitrary server from the connectString and attempt to connect to it. If that fails, is will continue
	  trying different random servers in round robin. (see above the algorithm used to initially choose a server)
    </p>
</div>

  
<a name="ch_zkWatches"></a>
<h2 class="h3">ZooKeeper Watches</h2>
<div class="section">
<p>All of the read operations in ZooKeeper - <strong>getData()</strong>, <strong>getChildren()</strong>, and <strong>exists()</strong> - have the option of setting a watch as a
    side effect. Here is ZooKeeper's definition of a watch: a watch event is
    one-time trigger, sent to the client that set the watch, which occurs when
    the data for which the watch was set changes. There are three key points
    to consider in this definition of a watch:</p>
<ul>
      
<li>
        
<p>
<strong>One-time trigger</strong>
</p>

        
<p>One watch event will be sent to the client when the data has changed.
        For example, if a client does a getData("/znode1", true) and later the
        data for /znode1 is changed or deleted, the client will get a watch
        event for /znode1. If /znode1 changes again, no watch event will be
        sent unless the client has done another read that sets a new
        watch.</p>
      
</li>

      
<li>
        
<p>
<strong>Sent to the client</strong>
</p>

        
<p>This implies that an event is on the way to the client, but may
        not reach the client before the successful return code to the change
        operation reaches the client that initiated the change. Watches are
        sent asynchronously to watchers. ZooKeeper provides an ordering
        guarantee: a client will never see a change for which it has set a
        watch until it first sees the watch event. Network delays or other
        factors may cause different clients to see watches and return codes
        from updates at different times. The key point is that everything seen
        by the different clients will have a consistent order.</p>
      
</li>

      
<li>
        
<p>
<strong>The data for which the watch was
        set</strong>
</p>

        
<p>This refers to the different ways a node can change.  It
        helps to think of ZooKeeper as maintaining two lists of
        watches: data watches and child watches.  getData() and
        exists() set data watches. getChildren() sets child
        watches. Alternatively, it may help to think of watches being
        set according to the kind of data returned. getData() and
        exists() return information about the data of the node,
        whereas getChildren() returns a list of children.  Thus,
        setData() will trigger data watches for the znode being set
        (assuming the set is successful). A successful create() will
        trigger a data watch for the znode being created and a child
        watch for the parent znode. A successful delete() will trigger
        both a data watch and a child watch (since there can be no
        more children) for a znode being deleted as well as a child
        watch for the parent znode.</p>
      
</li>
    
</ul>
<p>Watches are maintained locally at the ZooKeeper server to which the
    client is connected. This allows watches to be lightweight to set,
    maintain, and dispatch. When a client connects to a new server, the watch
    will be triggered for any session events. Watches will not be received
    while disconnected from a server. When a client reconnects, any previously
    registered watches will be reregistered and triggered if needed. In
    general this all occurs transparently. There is one case where a watch
    may be missed: a watch for the existence of a znode not yet created will
    be missed if the znode is created and deleted while disconnected.</p>
<a name="sc_WatchSemantics"></a>
<h3 class="h4">Semantics of Watches</h3>
<p> We can set watches with the three calls that read the state of 
	  ZooKeeper: exists, getData, and getChildren. The following list details
	  the events that a watch can trigger and the calls that enable them:
	  </p>
<ul>
        
<li>
          
<p>
<strong>Created event:</strong>
</p>
          
<p>Enabled with a call to exists.</p>
        
</li>
        
        
<li>
          
<p>
<strong>Deleted event:</strong>
</p>
          
<p>Enabled with a call to exists, getData, and getChildren.</p>
        
</li>
        
        
<li>
          
<p>
<strong>Changed event:</strong>
</p>
          
<p>Enabled with a call to exists and getData.</p>
        
</li>
        
        
<li>
          
<p>
<strong>Child event:</strong>
</p>
          
<p>Enabled with a call to getChildren.</p>
        
</li>
      
</ul>
<a name="sc_WatchRemoval"></a>
<h3 class="h4">Remove Watches</h3>
<p>We can remove the watches registered on a znode with a call to 
      removeWatches. Also, a ZooKeeper client can remove watches locally even
      if there is no server connection by setting the local flag to true. The 
      following list details the events which will be triggered after the 
      successful watch removal.
      </p>
<ul>
        
<li>
          
<p>
<strong>Child Remove event:</strong>
</p>
          
<p>Watcher which was added with a call to getChildren.</p>
        
</li>
        
        
<li>
          
<p>
<strong>Data Remove event:</strong>
</p>
          
<p>Watcher which was added with a call to exists or getData.</p>
        
</li>
      
</ul>
<a name="sc_WatchGuarantees"></a>
<h3 class="h4">What ZooKeeper Guarantees about Watches</h3>
<p>With regard to watches, ZooKeeper maintains these
      guarantees:</p>
<ul>
        
<li>
          
<p>Watches are ordered with respect to other events, other
          watches, and asynchronous replies. The ZooKeeper client libraries
          ensures that everything is dispatched in order.</p>
        
</li>
      
</ul>
<ul>
        
<li>
          
<p>A client will see a watch event for a znode it is watching
          before seeing the new data that corresponds to that znode.</p>
        
</li>
      
</ul>
<ul>
        
<li>
          
<p>The order of watch events from ZooKeeper corresponds to the
          order of the updates as seen by the ZooKeeper service.</p>
        
</li>
      
</ul>
<a name="sc_WatchRememberThese"></a>
<h3 class="h4">Things to Remember about Watches</h3>
<ul>
        
<li>
          
<p>Watches are one time triggers; if you get a watch event and
          you want to get notified of future changes, you must set another
          watch.</p>
        
</li>
      
</ul>
<ul>
        
<li>
          
<p>Because watches are one time triggers and there is latency
          between getting the event and sending a new request to get a watch
          you cannot reliably see every change that happens to a node in
          ZooKeeper. Be prepared to handle the case where the znode changes
          multiple times between getting the event and setting the watch
          again. (You may not care, but at least realize it may
          happen.)</p>
        
</li>
      
</ul>
<ul>
        
<li>
          
<p>A watch object, or function/context pair, will only be
          triggered once for a given notification. For example, if the same
          watch object is registered for an exists and a getData call for the
          same file and that file is then deleted, the watch object would
          only be invoked once with the deletion notification for the file.
          </p>
        
</li>
      
</ul>
<ul>
        
<li>
          
<p>When you disconnect from a server (for example, when the
          server fails), you will not get any watches until the connection
          is reestablished. For this reason session events are sent to all
          outstanding watch handlers. Use session events to go into a safe
          mode: you will not be receiving events while disconnected, so your
          process should act conservatively in that mode.</p>
        
</li>
      
</ul>
</div>

  
<a name="sc_ZooKeeperAccessControl"></a>
<h2 class="h3">ZooKeeper access control using ACLs</h2>
<div class="section">
<p>ZooKeeper uses ACLs to control access to its znodes (the
    data nodes of a ZooKeeper data tree). The ACL implementation is
    quite similar to UNIX file access permissions: it employs
    permission bits to allow/disallow various operations against a
    node and the scope to which the bits apply. Unlike standard UNIX
    permissions, a ZooKeeper node is not limited by the three standard
    scopes for user (owner of the file), group, and world
    (other). ZooKeeper does not have a notion of an owner of a
    znode. Instead, an ACL specifies sets of ids and permissions that
    are associated with those ids.</p>
<p>Note also that an ACL pertains only to a specific znode. In
    particular it does not apply to children. For example, if
    <em>/app</em> is only readable by ip:172.16.16.1 and
    <em>/app/status</em> is world readable, anyone will
    be able to read <em>/app/status</em>; ACLs are not
    recursive.</p>
<p>ZooKeeper supports pluggable authentication schemes. Ids are
    specified using the form <em>scheme:expression</em>,
    where <em>scheme</em> is the authentication scheme
    that the id corresponds to. The set of valid expressions are defined
    by the scheme. For example, <em>ip:172.16.16.1</em> is
    an id for a host with the address <em>172.16.16.1</em>
    using the <em>ip</em> scheme, whereas <em>digest:bob:password</em>
    is an id for the user with the name of <em>bob</em> using
    the <em>digest</em> scheme.</p>
<p>When a client connects to ZooKeeper and authenticates
    itself, ZooKeeper associates all the ids that correspond to a
    client with the clients connection. These ids are checked against
    the ACLs of znodes when a clients tries to access a node. ACLs are
    made up of pairs of <em>(scheme:expression,
    perms)</em>. The format of
    the <em>expression</em> is specific to the scheme. For
    example, the pair <em>(ip:19.22.0.0/16, READ)</em>
    gives the <em>READ</em> permission to any clients with
    an IP address that starts with 19.22.</p>
<a name="sc_ACLPermissions"></a>
<h3 class="h4">ACL Permissions</h3>
<p>ZooKeeper supports the following permissions:</p>
<ul>
        
<li>
<p>
<strong>CREATE</strong>: you can create a child node</p>
</li>
        
<li>
<p>
<strong>READ</strong>: you can get data from a node and list its children.</p>
</li>
        
<li>
<p>
<strong>WRITE</strong>: you can set data for a node</p>
</li>
        
<li>
<p>
<strong>DELETE</strong>: you can delete a child node</p>
</li>
        
<li>
<p>
<strong>ADMIN</strong>: you can set permissions</p>
</li>
      
</ul>
<p>The <em>CREATE</em>
      and <em>DELETE</em> permissions have been broken out
      of the <em>WRITE</em> permission for finer grained
      access controls. The cases for <em>CREATE</em>
      and <em>DELETE</em> are the following:</p>
<p>You want A to be able to do a set on a ZooKeeper node, but
      not be able to <em>CREATE</em>
      or <em>DELETE</em> children.</p>
<p>
<em>CREATE</em>
      without <em>DELETE</em>: clients create requests by
      creating ZooKeeper nodes in a parent directory. You want all
      clients to be able to add, but only request processor can
      delete. (This is kind of like the APPEND permission for
      files.)</p>
<p>Also, the <em>ADMIN</em> permission is there
      since ZooKeeper doesn&rsquo;t have a notion of file owner. In some
      sense the <em>ADMIN</em> permission designates the
      entity as the owner. ZooKeeper doesn&rsquo;t support the LOOKUP
      permission (execute permission bit on directories to allow you
      to LOOKUP even though you can't list the directory). Everyone
      implicitly has LOOKUP permission. This allows you to stat a
      node, but nothing more. (The problem is, if you want to call
      zoo_exists() on a node that doesn't exist, there is no
      permission to check.)</p>
<a name="sc_BuiltinACLSchemes"></a>
<h4>Builtin ACL Schemes</h4>
<p>ZooKeeeper has the following built in schemes:</p>
<ul>
        
<li>
<p>
<strong>world</strong> has a
        single id, <em>anyone</em>, that represents
        anyone.</p>
</li>

        
<li>
<p>
<strong>auth</strong> is a special
        scheme which ignores any provided expression and instead uses the current user,
        credentials, and scheme. Any expression (whether <em>user</em> like with SASL
        authentication or <em>user:password</em> like with DIGEST authentication) provided is ignored
        by the ZooKeeper server when persisting the ACL. However, the expression must still be
        provided in the ACL because the ACL must match the form <em>scheme:expression:perms</em>.
        This scheme is provided as a convenience as it is a common use-case for
        a user to create a znode and then restrict access to that znode to only that user.
        If there is no authenticated user, setting an ACL with the auth scheme will fail.
        </p>
</li>

        
<li>
<p>
<strong>digest</strong> uses
        a <em>username:password</em> string to generate
        MD5 hash which is then used as an ACL ID
        identity. Authentication is done by sending
        the <em>username:password</em> in clear text. When
        used in the ACL the expression will be
        the <em>username:base64</em>
        encoded <em>SHA1</em>
        password <em>digest</em>.</p>
        
</li>

        
<li>
<p>
<strong>ip</strong> uses the
        client host IP as an ACL ID identity. The ACL expression is of
        the form <em>addr/bits</em> where the most
        significant <em>bits</em>
        of <em>addr</em> are matched against the most
        significant <em>bits</em> of the client host
        IP.</p>
</li>

        
<li>
<p>
<strong>x509</strong> uses the client
        X500 Principal as an ACL ID identity. The ACL expression is the exact
        X500 Principal name of a client. When using the secure port, clients
        are automatically authenticated and their auth info for the x509 scheme
        is set.</p>
</li>

      
</ul>
<a name="ZooKeeper+C+client+API"></a>
<h4>ZooKeeper C client API</h4>
<p>The following constants are provided by the ZooKeeper C
      library:</p>
<ul>
        
<li>
<p>
<em>const</em> <em>int</em> ZOO_PERM_READ; //can read node&rsquo;s value and list its children</p>
</li>
        
<li>
<p>
<em>const</em> <em>int</em> ZOO_PERM_WRITE;// can set the node&rsquo;s value</p>
</li>
        
<li>
<p>
<em>const</em> <em>int</em> ZOO_PERM_CREATE; //can create children</p>
</li>
        
<li>
<p>
<em>const</em> <em>int</em> ZOO_PERM_DELETE;// can delete children</p>
</li>
        
<li>
<p>
<em>const</em> <em>int</em> ZOO_PERM_ADMIN; //can execute set_acl()</p>
</li>
        
<li>
<p>
<em>const</em> <em>int</em> ZOO_PERM_ALL;// all of the above flags OR&rsquo;d together</p>
</li>
      
</ul>
<p>The following are the standard ACL IDs:</p>
<ul>
        
<li>
<p>
<em>struct</em> Id ZOO_ANYONE_ID_UNSAFE; //(&lsquo;world&rsquo;,&rsquo;anyone&rsquo;)</p>
</li>
        
<li>
<p>
<em>struct</em> Id ZOO_AUTH_IDS;// (&lsquo;auth&rsquo;,&rsquo;&rsquo;)</p>
</li>
      
</ul>
<p>ZOO_AUTH_IDS empty identity string should be interpreted as &ldquo;the identity of the creator&rdquo;.</p>
<p>ZooKeeper client comes with three standard ACLs:</p>
<ul>
        
<li>
<p>
<em>struct</em> ACL_vector ZOO_OPEN_ACL_UNSAFE; //(ZOO_PERM_ALL,ZOO_ANYONE_ID_UNSAFE)</p>
</li>
        
<li>
<p>
<em>struct</em> ACL_vector ZOO_READ_ACL_UNSAFE;// (ZOO_PERM_READ, ZOO_ANYONE_ID_UNSAFE)</p>
</li>
        
<li>
<p>
<em>struct</em> ACL_vector ZOO_CREATOR_ALL_ACL; //(ZOO_PERM_ALL,ZOO_AUTH_IDS)</p>
</li>
      
</ul>
<p>The ZOO_OPEN_ACL_UNSAFE is completely open free for all
      ACL: any application can execute any operation on the node and
      can create, list and delete its children. The
      ZOO_READ_ACL_UNSAFE is read-only access for any
      application. CREATE_ALL_ACL grants all permissions to the
      creator of the node. The creator must have been authenticated by
      the server (for example, using &ldquo;<em>digest</em>&rdquo;
      scheme) before it can create nodes with this ACL.</p>
<p>The following ZooKeeper operations deal with ACLs:</p>
<ul>
<li>
          
<p>
<em>int</em> <em>zoo_add_auth</em>
            (zhandle_t *zh,<em>const</em> <em>char</em>*
            scheme,<em>const</em> <em>char</em>*
            cert, <em>int</em> certLen, void_completion_t
            completion, <em>const</em> <em>void</em>
            *data);</p>
      
</li>
</ul>
<p>The application uses the zoo_add_auth function to
      authenticate itself to the server. The function can be called
      multiple times if the application wants to authenticate using
      different schemes and/or identities.</p>
<ul>
<li>
          
<p>
<em>int</em> <em>zoo_create</em>
            (zhandle_t *zh, <em>const</em> <em>char</em>
            *path, <em>const</em> <em>char</em>
            *value,<em>int</em>
            valuelen, <em>const</em> <em>struct</em>
            ACL_vector *acl, <em>int</em>
            flags,<em>char</em>
            *realpath, <em>int</em>
            max_realpath_len);</p>
      
</li>
</ul>
<p>zoo_create(...) operation creates a new node. The acl
      parameter is a list of ACLs associated with the node. The parent
      node must have the CREATE permission bit set.</p>
<ul>
<li>
          
<p>
<em>int</em> <em>zoo_get_acl</em>
            (zhandle_t *zh, <em>const</em> <em>char</em>
            *path,<em>struct</em> ACL_vector
            *acl, <em>struct</em> Stat *stat);</p>
      
</li>
</ul>
<p>This operation returns a node&rsquo;s ACL info.</p>
<ul>
<li>
          
<p>
<em>int</em> <em>zoo_set_acl</em>
            (zhandle_t *zh, <em>const</em> <em>char</em>
            *path, <em>int</em>
            version,<em>const</em> <em>struct</em>
            ACL_vector *acl);</p>
      
</li>
</ul>
<p>This function replaces node&rsquo;s ACL list with a new one. The
      node must have the ADMIN permission set.</p>
<p>Here is a sample code that makes use of the above APIs to
      authenticate itself using the &ldquo;<em>foo</em>&rdquo; scheme
      and create an ephemeral node &ldquo;/xyz&rdquo; with create-only
      permissions.</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
<p>This is a very simple example which is intended to show
        how to interact with ZooKeeper ACLs
        specifically. See <span class="codefrag filename">.../trunk/src/c/src/cli.c</span>
        for an example of a C client implementation</p>
      
</div>
</div>
<pre class="code">
#include &lt;string.h&gt;
#include &lt;errno.h&gt;

#include "zookeeper.h"

static zhandle_t *zh;

/**
 * In this example this method gets the cert for your
 *   environment -- you must provide
 */
char *foo_get_cert_once(char* id) { return 0; }

/** Watcher function -- empty for this example, not something you should
 * do in real code */
void watcher(zhandle_t *zzh, int type, int state, const char *path,
             void *watcherCtx) {}

int main(int argc, char argv) {
  char buffer[512];
  char p[2048];
  char *cert=0;
  char appId[64];

  strcpy(appId, "example.foo_test");
  cert = foo_get_cert_once(appId);
  if(cert!=0) {
    fprintf(stderr,
            "Certificate for appid [%s] is [%s]\n",appId,cert);
    strncpy(p,cert, sizeof(p)-1);
    free(cert);
  } else {
    fprintf(stderr, "Certificate for appid [%s] not found\n",appId);
    strcpy(p, "dummy");
  }

  zoo_set_debug_level(ZOO_LOG_LEVEL_DEBUG);

  zh = zookeeper_init("localhost:3181", watcher, 10000, 0, 0, 0);
  if (!zh) {
    return errno;
  }
  if(zoo_add_auth(zh,"foo",p,strlen(p),0,0)!=ZOK)
    return 2;

  struct ACL CREATE_ONLY_ACL[] = {{ZOO_PERM_CREATE, ZOO_AUTH_IDS}};
  struct ACL_vector CREATE_ONLY = {1, CREATE_ONLY_ACL};
  int rc = zoo_create(zh,"/xyz","value", 5, &amp;CREATE_ONLY, ZOO_EPHEMERAL,
                      buffer, sizeof(buffer)-1);

  /** this operation will fail with a ZNOAUTH error */
  int buflen= sizeof(buffer);
  struct Stat stat;
  rc = zoo_get(zh, "/xyz", 0, buffer, &amp;buflen, &amp;stat);
  if (rc) {
    fprintf(stderr, "Error %d for %s\n", rc, __LINE__);
  }

  zookeeper_close(zh);
  return 0;
}
      </pre>
</div>

  
<a name="sc_ZooKeeperPluggableAuthentication"></a>
<h2 class="h3">Pluggable ZooKeeper authentication</h2>
<div class="section">
<p>ZooKeeper runs in a variety of different environments with
    various different authentication schemes, so it has a completely
    pluggable authentication framework. Even the builtin authentication
    schemes use the pluggable authentication framework.</p>
<p>To understand how the authentication framework works, first you must
    understand the two main authentication operations. The framework 
    first must authenticate the client. This is usually done as soon as
    the client connects to a server and consists of validating information
    sent from or gathered about a client and associating it with the connection.
    The second operation handled by the framework is finding the entries in an
    ACL that correspond to client. ACL entries are &lt;<em>idspec, 
    permissions</em>&gt; pairs. The <em>idspec</em> may be
    a simple string match against the authentication information associated
    with the connection or it may be a expression that is evaluated against that
    information. It is up to the implementation of the authentication plugin
    to do the match. Here is the interface that an authentication plugin must
    implement:</p>
<pre class="code">
public interface AuthenticationProvider {
    String getScheme();
    KeeperException.Code handleAuthentication(ServerCnxn cnxn, byte authData[]);
    boolean isValid(String id);
    boolean matches(String id, String aclExpr);
    boolean isAuthenticated();
}
    </pre>
<p>The first method <em>getScheme</em> returns the string
    that identifies the plugin. Because we support multiple methods of authentication,
    an authentication credential or an <em>idspec</em> will always be
    prefixed with <em>scheme:</em>. The ZooKeeper server uses the scheme
    returned by the authentication plugin to determine which ids the scheme
    applies to.</p>
<p>
<em>handleAuthentication</em> is called when a client
    sends authentication information to be associated with a connection. The
    client specifies the scheme to which the information corresponds. The
    ZooKeeper server passes the information to the authentication plugin whose
    <em>getScheme</em> matches the scheme passed by the client. The
    implementor of <em>handleAuthentication</em> will usually return
    an error if it determines that the information is bad, or it will associate information
    with the connection using <em>cnxn.getAuthInfo().add(new Id(getScheme(), data))</em>.
    </p>
<p>The authentication plugin is involved in both setting and using ACLs. When an
    ACL is set for a znode, the ZooKeeper server will pass the id part of the entry to
    the <em>isValid(String id)</em> method. It is up to the plugin to verify
    that the id has a correct form. For example, <em>ip:172.16.0.0/16</em>
    is a valid id, but <em>ip:host.com</em> is not. If the new ACL includes
    an "auth" entry, <em>isAuthenticated</em> is used to see if the 
    authentication information for this scheme that is assocatied with the connection
    should be added to the ACL. Some schemes
    should not be included in auth. For example, the IP address of the client is not
    considered as an id that should be added to the ACL if auth is specified.</p>
<p>ZooKeeper invokes
    <em>matches(String id, String aclExpr)</em> when checking an ACL. It
    needs to match authentication information of the client against the relevant ACL
    entries. To find the entries which apply to the client, the ZooKeeper server will
    find the scheme of each entry and if there is authentication information
    from that client for that scheme, <em>matches(String id, String aclExpr)</em>
    will be called with <em>id</em> set to the authentication information
    that was previously added to the connection by <em>handleAuthentication</em> and
    <em>aclExpr</em> set to the id of the ACL entry. The authentication plugin
    uses its own logic and matching scheme to determine if <em>id</em> is included
    in <em>aclExpr</em>. 
    </p>
<p>There are two built in authentication plugins: <em>ip</em> and
    <em>digest</em>. Additional plugins can adding using system properties. At
    startup the ZooKeeper server will look for system properties that start with
    "zookeeper.authProvider." and interpret the value of those properties as the class name
    of an authentication plugin. These properties can be set using the
    <em>-Dzookeeeper.authProvider.X=com.f.MyAuth</em> or adding entries such as
    the following in the server configuration file:</p>
<pre class="code">
authProvider.1=com.f.MyAuth
authProvider.2=com.f.MyAuth2
    </pre>
<p>Care should be taking to ensure that the suffix on the property is unique. If there are 
    duplicates such as <em>-Dzookeeeper.authProvider.X=com.f.MyAuth -Dzookeeper.authProvider.X=com.f.MyAuth2</em>,
    only one will be used. Also all servers must have the same plugins defined, otherwise clients using
    the authentication schemes provided by the plugins will have problems connecting to some servers.
    </p>
</div>
      
  
<a name="ch_zkGuarantees"></a>
<h2 class="h3">Consistency Guarantees</h2>
<div class="section">
<p>ZooKeeper is a high performance, scalable service. Both reads and
    write operations are designed to be fast, though reads are faster than
    writes. The reason for this is that in the case of reads, ZooKeeper can
    serve older data, which in turn is due to ZooKeeper's consistency
    guarantees:</p>
<dl>
      
<dt>
<term>Sequential Consistency</term>
</dt>
<dd>
<p>Updates from a client will be applied in the order that they
          were sent.</p>
</dd>

      
<dt>
<term>Atomicity</term>
</dt>
<dd>
<p>Updates either succeed or fail -- there are no partial
          results.</p>
</dd>

      
<dt>
<term>Single System Image</term>
</dt>
<dd>
<p>A client will see the same view of the service regardless of
          the server that it connects to.</p>
</dd>

      
<dt>
<term>Reliability</term>
</dt>
<dd>
<p>Once an update has been applied, it will persist from that
          time forward until a client overwrites the update. This guarantee
          has two corollaries:</p>
<ol>
            
<li>
              
<p>If a client gets a successful return code, the update will
              have been applied. On some failures (communication errors,
              timeouts, etc) the client will not know if the update has
              applied or not. We take steps to minimize the failures, but the
              guarantee is only present with successful return codes.
              (This is called the <em>monotonicity condition</em> in Paxos.)</p>
            
</li>

            
<li>
              
<p>Any updates that are seen by the client, through a read
              request or successful update, will never be rolled back when
              recovering from server failures.</p>
            
</li>
          
</ol>
</dd>

      
<dt>
<term>Timeliness</term>
</dt>
<dd>
<p>The clients view of the system is guaranteed to be up-to-date
          within a certain time bound (on the order of tens of seconds).
          Either system changes will be seen by a client within this bound, or
          the client will detect a service outage.</p>
</dd>
    
</dl>
<p>Using these consistency guarantees it is easy to build higher level
    functions such as leader election, barriers, queues, and read/write
    revocable locks solely at the ZooKeeper client (no additions needed to
    ZooKeeper). See <a href="recipes.html">Recipes and Solutions</a>
    for more details.</p>
<div class="note">
<div class="label">Note</div>
<div class="content">
        
<p>Sometimes developers mistakenly assume one other guarantee that
        ZooKeeper does <em>not</em> in fact make. This is:</p>

        
<dl>
          
<dt>
<term>Simultaneously Consistent Cross-Client Views</term>
</dt>
<dd>
<p>ZooKeeper does not guarantee that at every instance in
              time, two different clients will have identical views of
              ZooKeeper data. Due to factors like network delays, one client
              may perform an update before another client gets notified of the
              change. Consider the scenario of two clients, A and B. If client
              A sets the value of a znode /a from 0 to 1, then tells client B
              to read /a, client B may read the old value of 0, depending on
              which server it is connected to. If it
              is important that Client A and Client B read the same value,
              Client B should should call the <strong>sync()</strong> method from the ZooKeeper API
              method before it performs its read.</p>
<p>So, ZooKeeper by itself doesn't guarantee that changes occur 
              synchronously across all servers, but ZooKeeper
              primitives can be used to construct higher level functions that
              provide useful client synchronization. (For more information,
              see the <a href="recipes.html">ZooKeeper Recipes</a>.
              <em>[tbd:..]</em>).</p>
</dd>
        
</dl>
      
</div>
</div>
</div>

  
<a name="ch_bindings"></a>
<h2 class="h3">Bindings</h2>
<div class="section">
<p>The ZooKeeper client libraries come in two languages: Java and C.
    The following sections describe these.</p>
<a name="Java+Binding"></a>
<h3 class="h4">Java Binding</h3>
<p>There are two packages that make up the ZooKeeper Java binding:
      <strong>org.apache.zookeeper</strong> and <strong>org.apache.zookeeper.data</strong>. The rest of the
      packages that make up ZooKeeper are used internally or are part of the
      server implementation. The <strong>org.apache.zookeeper.data</strong> package is made up of
      generated classes that are used simply as containers.</p>
<p>The main class used by a ZooKeeper Java client is the <strong>ZooKeeper</strong> class. Its two constructors differ only
      by an optional session id and password. ZooKeeper supports session
      recovery accross instances of a process. A Java program may save its
      session id and password to stable storage, restart, and recover the
      session that was used by the earlier instance of the program.</p>
<p>When a ZooKeeper object is created, two threads are created as
      well: an IO thread and an event thread. All IO happens on the IO thread
      (using Java NIO). All event callbacks happen on the event thread.
      Session maintenance such as reconnecting to ZooKeeper servers and
      maintaining heartbeat is done on the IO thread. Responses for
      synchronous methods are also processed in the IO thread. All responses
      to asynchronous methods and watch events are processed on the event
      thread. There are a few things to notice that result from this
      design:</p>
<ul>
        
<li>
          
<p>All completions for asynchronous calls and watcher callbacks
          will be made in order, one at a time. The caller can do any
          processing they wish, but no other callbacks will be processed
          during that time.</p>
        
</li>

        
<li>
          
<p>Callbacks do not block the processing of the IO thread or the
          processing of the synchronous calls.</p>
        
</li>

        
<li>
          
<p>Synchronous calls may not return in the correct order. For
          example, assume a client does the following processing: issues an
          asynchronous read of node <strong>/a</strong> with
          <em>watch</em> set to true, and then in the completion
          callback of the read it does a synchronous read of <strong>/a</strong>. (Maybe not good practice, but not illegal
          either, and it makes for a simple example.)</p>

          
<p>Note that if there is a change to <strong>/a</strong> between the asynchronous read and the
          synchronous read, the client library will receive the watch event
          saying <strong>/a</strong> changed before the
          response for the synchronous read, but because the completion
          callback is blocking the event queue, the synchronous read will
          return with the new value of <strong>/a</strong>
          before the watch event is processed.</p>
        
</li>
      
</ul>
<p>Finally, the rules associated with shutdown are straightforward:
      once a ZooKeeper object is closed or receives a fatal event
      (SESSION_EXPIRED and AUTH_FAILED), the ZooKeeper object becomes invalid.
      On a close, the two threads shut down and any further access on zookeeper
      handle is undefined behavior and should be avoided. </p>
<a name="sc_java_client_configuration"></a>
<h4>Client Configuration Parameters</h4>
<p>
            The following list contains configuration properties for the Java client. You can set any
            of these properties using Java system properties. For server properties, please check the
            following reference
            <a href="zookeeperAdmin.html#sc_configuration">Server configuration section.</a>
        
</p>
<dl>
            
<dt>
<term>zookeeper.sasl.client</term>
</dt>
<dd>
<p>Set the value to <strong>false</strong> to disable
                    SASL authentication. Default is <strong>true</strong>.</p>
</dd>
            
<dt>
<term>zookeeper.sasl.clientconfig</term>
</dt>
<dd>
<p>Specifies the context key in the JAAS login file. Default is "Client".</p>
</dd>
            
<dt>
<term>zookeeper.sasl.client.username</term>
</dt>
<dd>
<p>Traditionally, a principal is divided into three parts: the primary, the instance, and the realm.
                        The format of a typical Kerberos V5 principal is primary/instance@REALM.
                        zookeeper.sasl.client.username specifies the primary part of the server principal. Default
                        is "zookeeper". Instance part is derived from the server IP. Finally server's principal is
                        username/IP@realm, where username is the value of zookeeper.sasl.client.username, IP is
                        the server IP, and realm is the value of zookeeper.server.realm.</p>
</dd>
            
<dt>
<term>zookeeper.server.realm</term>
</dt>
<dd>
<p>Realm part of the server principal. By default it is the client principal realm.</p>
</dd>
            
<dt>
<term>zookeeper.disableAutoWatchReset</term>
</dt>
<dd>
<p>This switch controls whether automatic watch resetting is enabled. Clients automatically
                        reset watches during session reconnect by default, this option allows the client to turn off
                        this behavior by setting zookeeper.disableAutoWatchReset to <strong>true</strong>. 
                    </p>
</dd>
            
<dt>
<term>zookeeper.client.secure</term>
</dt>
<dd>
<p>
                        If you want to connect to the server secure client port, you need to set this property to
                        <strong>true</strong>
                        on the client. This will connect to server using SSL with specified credentials. Note that
                        it requires the Netty client.
                    </p>
</dd>
            
<dt>
<term>zookeeper.clientCnxnSocket</term>
</dt>
<dd>
<p>
                        Specifies which ClientCnxnSocket to be used. Possible values are
                        <strong>org.apache.zookeeper.ClientCnxnSocketNIO</strong>
                        and
                        <strong>org.apache.zookeeper.ClientCnxnSocketNetty</strong>
                        . Default is
                        <strong>org.apache.zookeeper.ClientCnxnSocketNIO</strong>
                        . If you want to connect to server's secure client port, you need to set this property to
                        <strong>org.apache.zookeeper.ClientCnxnSocketNetty</strong>
                        on client.
                    </p>
</dd>
            
<dt>
<term>zookeeper.ssl.keyStore.location and zookeeper.ssl.keyStore.password</term>
</dt>
<dd>
<p>Specifies the file path to a JKS containing the local credentials to be used for SSL connections,
                        and the password to unlock the file.
                    </p>
</dd>
            
<dt>
<term>zookeeper.ssl.trustStore.location and zookeeper.ssl.trustStore.password</term>
</dt>
<dd>
<p>Specifies the file path to a JKS containing the remote credentials to be used for SSL connections,
                        and the password to unlock the file.
                    </p>
</dd>
            
<dt>
<term>jute.maxbuffer</term>
</dt>
<dd>
<p>It specifies the maximum size of the incoming data from the server. The default value is 4194304
                        Bytes , or just 4 MB. This is really a sanity check. The ZooKeeper server is designed to store and send
                        data on the order of kilobytes. If incoming data length is more than this value, an IOException
                        is raised.</p>
</dd>
            
<dt>
<term>zookeeper.kinit</term>
</dt>
<dd>
<p>Specifies path to kinit binary. Default is "/usr/bin/kinit".</p>
</dd>
        
</dl>
<a name="C+Binding"></a>
<h3 class="h4">C Binding</h3>
<p>The C binding has a single-threaded and multi-threaded library.
      The multi-threaded library is easiest to use and is most similar to the
      Java API. This library will create an IO thread and an event dispatch
      thread for handling connection maintenance and callbacks. The
      single-threaded library allows ZooKeeper to be used in event driven
      applications by exposing the event loop used in the multi-threaded
      library.</p>
<p>The package includes two shared libraries: zookeeper_st and
      zookeeper_mt. The former only provides the asynchronous APIs and
      callbacks for integrating into the application's event loop. The only
      reason this library exists is to support the platforms were a
      <em>pthread</em> library is not available or is unstable
      (i.e. FreeBSD 4.x). In all other cases, application developers should
      link with zookeeper_mt, as it includes support for both Sync and Async
      API.</p>
<a name="Installation"></a>
<h4>Installation</h4>
<p>If you're building the client from a check-out from the Apache
        repository, follow the steps outlined below. If you're building from a
        project source package downloaded from apache, skip to step <strong>3</strong>.</p>
<ol>
          
<li>
            
<p>Run <span class="codefrag command">ant compile_jute</span> from the ZooKeeper
            top level directory (<span class="codefrag filename">.../trunk</span>).
            This will create a directory named "generated" under
            <span class="codefrag filename">.../trunk/src/c</span>.</p>
          
</li>

          
<li>
            
<p>Change directory to the<span class="codefrag filename">.../trunk/src/c</span>
            and run <span class="codefrag command">autoreconf -if</span> to bootstrap <strong>autoconf</strong>, <strong>automake</strong> and <strong>libtool</strong>. Make sure you have <strong>autoconf version 2.59</strong> or greater installed.
            Skip to step<strong> 4</strong>.</p>
          
</li>

          
<li>
            
<p>If you are building from a project source package,
            unzip/untar the source tarball and cd to the<span class="codefrag filename">
            zookeeper-x.x.x/src/c</span> directory.</p>
          
</li>

          
<li>
            
<p>Run <span class="codefrag command">./configure &lt;your-options&gt;</span> to
            generate the makefile. Here are some of options the <strong>configure</strong> utility supports that can be
            useful in this step:</p>

            
<ul>
              
<li>
                
<p>
<span class="codefrag command">--enable-debug</span>
</p>

                
<p>Enables optimization and enables debug info compiler
                options. (Disabled by default.)</p>
              
</li>

              
<li>
                
<p>
<span class="codefrag command">--without-syncapi </span>
</p>

                
<p>Disables Sync API support; zookeeper_mt library won't be
                built. (Enabled by default.)</p>
              
</li>

              
<li>
                
<p>
<span class="codefrag command">--disable-static </span>
</p>

                
<p>Do not build static libraries. (Enabled by
                default.)</p>
              
</li>

              
<li>
                
<p>
<span class="codefrag command">--disable-shared</span>
</p>

                
<p>Do not build shared libraries. (Enabled by
                default.)</p>
              
</li>
            
</ul>

            
<div class="note">
<div class="label">Note</div>
<div class="content">
              
<p>See INSTALL for general information about running
              <strong>configure</strong>.</p>
            
</div>
</div>
          
</li>

          
<li>
            
<p>Run <span class="codefrag command">make</span> or <span class="codefrag command">make
            install</span> to build the libraries and install them.</p>
          
</li>

          
<li>
            
<p>To generate doxygen documentation for the ZooKeeper API, run
            <span class="codefrag command">make doxygen-doc</span>. All documentation will be
            placed in a new subfolder named docs. By default, this command
            only generates HTML. For information on other document formats,
            run <span class="codefrag command">./configure --help</span>
</p>
          
</li>
        
</ol>
<a name="Building+Your+Own+C+Client"></a>
<h4>Building Your Own C Client</h4>
<p>In order to be able to use the ZooKeeper C API in your application
        you have to remember to</p>
<ol>
          
<li>
            
<p>Include ZooKeeper header: #include
              &lt;zookeeper/zookeeper.h&gt;</p>
          
</li>

          
<li>
            
<p>If you are building a multithreaded client, compile with
            -DTHREADED compiler flag to enable the multi-threaded version of
            the library, and then link against against the
            <em>zookeeper_mt</em> library. If you are building a
            single-threaded client, do not compile with -DTHREADED, and be
            sure to link against the<em> zookeeper_st
            </em>library.</p>
          
</li>
        
</ol>
<div class="note">
<div class="label">Note</div>
<div class="content">
<p>See <span class="codefrag filename">.../trunk/src/c/src/cli.c</span>
          for an example of a C client implementation</p>
        
</div>
</div>
</div>

   
<a name="ch_guideToZkOperations"></a>
<h2 class="h3">Building Blocks: A Guide to ZooKeeper Operations</h2>
<div class="section">
<p>This section surveys all the operations a developer can perform
    against a ZooKeeper server. It is lower level information than the earlier
    concepts chapters in this manual, but higher level than the ZooKeeper API
    Reference. It covers these topics:</p>
<ul>
      
<li>
        
<p>
<a href="#sc_connectingToZk">Connecting to ZooKeeper</a>
</p>
      
</li>
    
</ul>
<a name="sc_errorsZk"></a>
<h3 class="h4">Handling Errors</h3>
<p>Both the Java and C client bindings may report errors. The Java client binding does so by throwing KeeperException, calling code() on the exception will return the specific error code. The C client binding returns an error code as defined in the enum ZOO_ERRORS. API callbacks indicate result code for both language bindings. See the API documentation (javadoc for Java, doxygen for C) for full details on the possible errors and their meaning.</p>
<a name="sc_connectingToZk"></a>
<h3 class="h4">Connecting to ZooKeeper</h3>
<p></p>
<a name="sc_readOps"></a>
<h3 class="h4">Read Operations</h3>
<p></p>
<a name="sc_writeOps"></a>
<h3 class="h4">Write Operations</h3>
<p></p>
<a name="sc_handlingWatches"></a>
<h3 class="h4">Handling Watches</h3>
<p></p>
<a name="sc_miscOps"></a>
<h3 class="h4">Miscelleaneous ZooKeeper Operations</h3>
<p></p>
</div>

  
<a name="ch_programStructureWithExample"></a>
<h2 class="h3">Program Structure, with Simple Example</h2>
<div class="section">
<p>
<em>[tbd]</em>
</p>
</div>

  
<a name="ch_gotchas"></a>
<h2 class="h3">Gotchas: Common Problems and Troubleshooting</h2>
<div class="section">
<p>So now you know ZooKeeper. It's fast, simple, your application
    works, but wait ... something's wrong. Here are some pitfalls that
    ZooKeeper users fall into:</p>
<ol>
      
<li>
        
<p>If you are using watches, you must look for the connected watch
        event. When a ZooKeeper client disconnects from a server, you will
        not receive notification of changes until reconnected. If you are
        watching for a znode to come into existence, you will miss the event
        if the znode is created and deleted while you are disconnected.</p>
      
</li>

      
<li>
        
<p>You must test ZooKeeper server failures. The ZooKeeper service
        can survive failures as long as a majority of servers are active. The
        question to ask is: can your application handle it? In the real world
        a client's connection to ZooKeeper can break. (ZooKeeper server
        failures and network partitions are common reasons for connection
        loss.) The ZooKeeper client library takes care of recovering your
        connection and letting you know what happened, but you must make sure
        that you recover your state and any outstanding requests that failed.
        Find out if you got it right in the test lab, not in production - test
        with a ZooKeeper service made up of a several of servers and subject
        them to reboots.</p>
      
</li>

      
<li>
        
<p>The list of ZooKeeper servers used by the client must match the
        list of ZooKeeper servers that each ZooKeeper server has. Things can
        work, although not optimally, if the client list is a subset of the
        real list of ZooKeeper servers, but not if the client lists ZooKeeper
        servers not in the ZooKeeper cluster.</p>
      
</li>

      
<li>
        
<p>Be careful where you put that transaction log. The most
        performance-critical part of ZooKeeper is the transaction log.
        ZooKeeper must sync transactions to media before it returns a
        response. A dedicated transaction log device is key to consistent good
        performance. Putting the log on a busy device will adversely effect
        performance. If you only have one storage device, put trace files on
        NFS and increase the snapshotCount; it doesn't eliminate the problem,
        but it can mitigate it.</p>
      
</li>

      
<li>
        
<p>Set your Java max heap size correctly. It is very important to
        <em>avoid swapping.</em> Going to disk unnecessarily will
        almost certainly degrade your performance unacceptably. Remember, in
        ZooKeeper, everything is ordered, so if one request hits the disk, all
        other queued requests hit the disk.</p>

        
<p>To avoid swapping, try to set the heapsize to the amount of
        physical memory you have, minus the amount needed by the OS and cache.
        The best way to determine an optimal heap size for your configurations
        is to <em>run load tests</em>. If for some reason you
        can't, be conservative in your estimates and choose a number well
        below the limit that would cause your machine to swap. For example, on
        a 4G machine, a 3G heap is a conservative estimate to start
        with.</p>
      
</li>
    
</ol>
</div>

  
<a name="apx_linksToOtherInfo"></a>
<appendix id="apx_linksToOtherInfo">
    
<title>Links to Other Information</title>

    
<p>Outside the formal documentation, there're several other sources of
    information for ZooKeeper developers.</p>

    
<dl>
      
<dt>
<term>ZooKeeper Whitepaper <em>[tbd: find url]</em>
</term>
</dt>
<dd>
<p>The definitive discussion of ZooKeeper design and performance,
          by Yahoo! Research</p>
</dd>

      
<dt>
<term>API Reference <em>[tbd: find url]</em>
</term>
</dt>
<dd>
<p>The complete reference to the ZooKeeper API</p>
</dd>

      
<dt>
<term>
<a href="http://us.dl1.yimg.com/download.yahoo.com/dl/ydn/zookeeper.m4v">ZooKeeper
        Talk at the Hadoup Summit 2008</a>
</term>
</dt>
<dd>
<p>A video introduction to ZooKeeper, by Benjamin Reed of Yahoo!
          Research</p>
</dd>

      
<dt>
<term>
<a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Tutorial">Barrier and
        Queue Tutorial</a>
</term>
</dt>
<dd>
<p>The excellent Java tutorial by Flavio Junqueira, implementing
          simple barriers and producer-consumer queues using ZooKeeper.</p>
</dd>

      
<dt>
<term>
<a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/ZooKeeperArticles">ZooKeeper
        - A Reliable, Scalable Distributed Coordination System</a>
</term>
</dt>
<dd>
<p>An article by Todd Hoff (07/15/2008)</p>
</dd>

      
<dt>
<term>
<a href="recipes.html">ZooKeeper Recipes</a>
</term>
</dt>
<dd>
<p>Pseudo-level discussion of the implementation of various
          synchronization solutions with ZooKeeper: Event Handles, Queues,
          Locks, and Two-phase Commits.</p>
</dd>

      
<dt>
<term>
<em>[tbd]</em>
</term>
</dt>
<dd>
<p>Any other good sources anyone can think of...</p>
</dd>
    
</dl>
  
</appendix>

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